1. Field of the Invention
The present invention relates, in general, to a method for producing hydrogen storage alloy electrodes for Ni/metal hydride (MH) secondary cells and, more particularly, to an improvement in the general functions of the cells without producing pollution of the environment, along with the method.
2. Description of the Prior Art
Hydrogen storage alloys are the metals or alloys which are able to absorb or discharge hydrogen reversibly at certain temperatures under certain pressures. In order for the hydrogen storage alloys to be applied in practice, they are required to show large hydrogen storage capacities which are reversibly available as well as to show rapid hydrogenation in electrolytes.
The hydrogen storage alloys for Ni/MH secondary cells, developed thus far, can be exemplified largely by two types: AB.sub.5 type including Mm-Ni, wherein A is an element having a high affinity for hydrogen, that is, a rare-earth element, such as La, Ce, Ti, Zr, etc, and B is a transition metal or transition metals selected from Ni, Mn, Co, Fe, Al, etc; and AB.sub.2 type including Zr-Ni and Ti-Ni. The former AB.sub.5 type is disadvantageous in that its energy storage density is low while the latter AB.sub.2 type is poor in its general functions.
In developing anode materials for Ni/MH cells, extensive research has been focused on the AB.sub.5 type hydrogen storage alloys and has resulted in MmNi.sub.5 type alloys with an electrochemical discharge capacity of about 200-300 mAh/g.
However, the miniaturization of electronic equipment requires alloys which are of higher discharge capacity and better electrode life span than the conventional MmNi.sub.5 type alloys. This requirement is also raised by the development of electric vehicles which demand high capacity and high performance batteries. To develop the alloys which meet the requirement, the research direction has recently turned toward AB.sub.2 type Laves phase alloys which are now known to be of higher discharge capacity than conventional AB.sub.5 type alloys.
Thus, in order to develop the Ni/NH secondary cells which are of high capacity and high performance, it is necessary to research for the high performance of the AB.sub.5 type hydrogen storage alloys for which higher capacity is secured than for the AB.sub.2 type hydrogen storage alloys.
All of an electrode's functions including activation properties, low temperature dischargeability, dependence on current density, and electrode's life span, were found to be greatly dependent on electrode-producing methods as well as the properties of hydrogen storage alloys as they are.
Improvement of the anodes for Ni/MH secondary cells is largely accomplished by amelioration of active materials themselves and/or by additives. The amelioration, which aims to maximize the properties the active materials themselves have, comprises the change in alloy composition (alloy design) and the modification of alloy surface through, for example, electroless plating. For the additives, current collectors, such as Cu and Ni, and binders, such as polytetrafluoroethylene (hereinafter referred to as "PTFE") and PVA powders, may be used.
T. Sakai et al., reported in J. Less-Common Metals, 172-174(1991) 1135 that The low temperature dischargeability and current density dependence of electrodes can be improved by electroless plating Ni or Cu on their surfaces. The electroless plating processes suggested by T. Sakai et al., however, are complicated and produce pollution of the environment owing to their toxic by-products.
A method for improving the general functions of electrodes by changing the properties that active materials themselves have, rather than by the additional processes, such as electroless plating, is disclosed in Mat. Trans, JIM, 31 (1990) 487 by H. Sawa et al. According to the disclosure, the content of Ni in a hydrogen storage alloy is increased from the start of the alloy's design. However, the discharge capacity of the electrode is found to decrease with Ni content.
Lee et al., reported in Ph. D. thesis, KAIST, Taejon, Korea (1995) that the surfaces of electroless-plated alloy powders are coated with Cu or Ni particles less than 10 .mu.m in size. This electroless plating can extend the life span of electrodes by preventing the electrodes from being in direct contact with electrolytes. Other significant advantages of the electroless plating are that the plated alloys are improved in moldability and electroconductivity. However, the functional improvement of the electrodes is difficult to control because the size of the plated particles, a main factor to determine the functions, is changed depending on the conditions of the plating processes.